Method of producing castings having high mechanical properties



United States Patent 2,887,421 METHOD OF'PRODUCINGCASTINGS HAVING HIGHMECHANICAL PROPERTIES "Lucien Pras, Billancourt, France, assignor toRegieNationale desUsines Renault, Billancourt, France No Drawing.Application December 27, 1955 .Serial No. 555,304

"Claims priority, application France January 5, 1955 3 Claims. (Cl.148--21.8)

TThepresent invention relates to a method of producing cast ironcastings having high mechanical properties, and constitutes animprovement in or modification of the "method described in the patentapplication Ser. Number 441,923 of July 7, 1954, now Patent No.2,791,526, issued May 7, 1957, in tlie name of same applicant.

The aforesaid patent relates to the total primary .graphitisation ofsulphur-containing white iron castings then to form the graphite nucleiby epistaxis on the hexagonal pyrites, and finally to permit freedevelopment of the nuclei to provide graphite spherules.

The process which permits in the case of cast iron, which is totallywhite upon the pouring operation, of a graphitization upon annealingwithout the formation of cementite produced by solidification, resultingin a totally spherulitic graphite, is characterized by the followingfeatures:

(1) The composition comprises a proportion of S/Mn which is greater than1-- and more particularly less than 0.15 manganese and more than 0.2sulphur, and preferably it is comprised within the following limits:

C=2.10 to 2.70 Si=0.85 to 1.10 Mn=().002 to 0.15 S=0.2O to 0.30 P=0.10

Generally speaking, there are not special components. (2) Solidificationin the mold should take place as rapidly as possible, and preferablymore rapidly than is strictly necessary to obtain the white structure.This leads to a fine distribution of the iron sulphide, on which theredepends the fineness of distribution of the graphite spherules obtainedduring the course of the subsequent annealing process. Casting in achill mold is preferable to casting in sand.

(3) The thermal treatment comprises essentially a martensite hardeningor, if desired, an inferior hardening before carrying out thegraphitization by annealing.

This hardening is performed with advantage in conditions of rapidcooling, preferably for example in oil, or if desired in a bath ofsalts, which may be a fusible mixture of alkaline nitrate and alkalinenitrite and leads to a martensitic structure.

To avoid the danger of shrinkage upon hardening there has beenrecognized according to the invention the prime importance of effectinga preliminary annealing, a simple austenization beyond termination ofthe transformation upon the heating A followed by cooling at a mediumspeed, such as cooling in still air.

It has also been recognized 'that if the cooling for the purpose ofsolidification ismoderate (casting in sand),

it may be necessary to carry out two consecutiveaustenization treatmentswith martensitic hardening.

(4) The thermaltreatment furthermore comprises, followingthe martensitichardening, a graphitization an nealing beyond the point A i.e., at atemperature sufiiciently high so that the casting will be constitutedonly by cementite and austenite, such temperature being maintainedfor asuificientlength of time to permit of total disappearance of thecementite not dissolved in the austenite at the annealing temperature.The duration of this annealing process is less than that in the case ofusual white iron malleable castings annealed at the same temperature.

The consecutive cooling is carried out in accordance withthe mechanicalproperties sought to be achieved. There may be obtained a totaleutectoidal graphitization in respect to a ferrite and graphitestructure, as in malleable castings of'the usual kindvery soft-orpreferably thegraphitization may be stopped at such a point that the.matrix has a substantially eutectoidal (lamellar perlite) composition,which leads to increased toughness.

Finally, .it is.possible as a subsequent step to carry outaustenization, quenching and tempering in order to obtain a greatertoughness.

An example of the above method is given in the following:

Pieces ofmetal having a weight of 700 grams and .a mean thickness of 20mm. are cast in molds which are partially metal and partially composedof sand.

The castings have the following composition:

Carbon=2.5 Silicon: 1 Manganese: 0.12 Sulphur=0.28 Phosphorus: 0.10

Following stripping and complete cooling of the castings the latter areheated to 820 over a period of 30 minutes and are cooled in still air.There is thereupon carried out a second heating to 825 over a period of30 minutes, and the pieces of metal are hardened in oil. The pieces arethen reheated to 940 for a period of eight hours, after which they areremoved from the furnace and cooled in still air.

A structure is obtained comprising fiine graphite spherules on alamellar perlite base.

Mechanical tests carried out on test pieces taken from these metal partshave produced the following results:

Tensile strength 55 to kg./mm. Breaking strength to kg./mm. Elongationlimit 5 to 6% Properties of this nature have not been obtainedcollectively heretofore in conjunction with castings devoid of allalloyed substances.

The limit of resistance to rotative bending has been measured inconjunction with Moore test pieces.

The resistance limit has been found to be 38 to 40 kg./mm. which isgreater than that of the usual spheroidal castings.

The method referred to has the following advantages:

It is applicable to readily produced castings which make use of orescontaining sulphur, and are of low commercial value, and it does notresort to any special substance alloyed therewith.

It provides, upon annealing, a very even distribution of graphite in theform of spherules, this structure being associated itself with very highmechanical properties, the breaking strength being between 95 and 1.05kg./mm. with an elongation of between 5 and 6.5% if the matrix islamellar perlitic.

According to the present invention, a modified method of treatmentconsists in heating white iron castings to a temperature of between 880and 950 C. over a period of time varying between 4 and l'h'ours. Forexample, an ordinary heating to 900 C. over a period of 9 hours followedby a cooling in still air brings about complete graphitisation of theprimary cementite. Micrographic examination shows that the graphite isproduced in the form of fine spherules on a lamellar pearlite base.

, According to a modification of the present invention graphitisationmay be effected by carrying out successive operations as follows: aheating followed by a quenching operation, which imparts to the metal amartensitic structure, a germination treatment eifected at temperatureslocated between 300 and 700 C. over a period of from 1 to 4 hours, andfinally a graphitisation heating opera tion, which may be effected at alower temperature and over a shorter period of time than in those casesin which there is carried out merely a preliminary quenching operation,or those cases in which graphitisation is performed directly withneither quenching nor germination. Total graphitisation of the primarycementite is obtained, for example, by a heating for 3 to 8 hours to atemperature of between 850 and 900 C.

Heating, 850 C., 30 minutes; oil hardening, Reheating, 470 C., 4 hours;cooling in still air, Reheating, 900 C., 6 hours; cooling in still air,

may be quoted as an example of this latter method of carrying out theprocess. A casting thus treated comprises fine, well-formed spherules ona lamellar pearlite base.

I claim:

1. In a process of thermally treating white castings to producemalleable iron with annealed spheroidal graphite therefrom, saidcastings containing 0.85 to 1.10 silicon, 0.2 to 0.3% sulfur and 0.002to 0.15 manganese with the ratio S/Mn being greater than 1, the stepswhich comprise heating said white castings at a temperature between 880and 950 C. for 4 to 10 hours, and cooling in still air.

2. In a process of thermally treating white castings to producemalleable iron with annealed spheroidal graphite therefrom, saidcastings containing 0.2 to 0.3% sulfur and 0.002 to 0.15 manganese withthe ratio S/Mn being greater than 1, said process comprising the stepsof heating said castings and directly quenching to impart a martensiticstructure to the metal, heating at 300 to 700 C. for 1 to 4 hours andgrhaphitizing by heating for 3 to 8 hours at a temperature of 850 to900C.

3. In a process of thermally treating white castings to producemalleable iron with annealed spheroidal graphite therefrom, saidcastings containing 0.85 to 1.10 silicon 0.2 to 0.3% sulfur and 0.002 to0.15 manganese with the ratio S/Mn being greater than 1, the step whichcomprises heating said white castings at 900 C. for 9 hours, and coolingin still air.

References Cited in the file of this patent UNITED STATES PATENTSDrysdale June 16, 1925 Hultgren Ian. 2, 1940 OTHER REFERENCES

2. IN A PROCESS OF THERMALLY TREATING WHITE CASTINGS TO PRODUCEMALLEABLE IRON WITH ANNEALED SPHEROIDAL GRAPHITE THEREFROM, SAIDCASTINGS CONTAINING 0.2 TO 0.3% SULFUR AND 0.002 TO 0.15 MANGANESE WITHTHE RATIO S/MN BEING GREATER THAN 1, SAID PROCESS COMPRISING THE STEPSOF HEATING SAID CASTINGS AND DIRECTLY QUENCHING TO IMPART A MARTENSITICSTRUCTURE TO THE METAL, HEATING AT 300* TO 700* C. FOR 1 TO 4 HOURS ANDGRHAPHITIZING BY HEATING FOR 3 TO 8 HOURS AT A TEMPERATURE OF 850* TO900* C.